It is no surprise that most work-related hearing loss is caused by noise exposure, and that genetics and age can also be contributors. What may not be as well known is that some chemical exposures can pose a potential risk to hearing. Both animal experiments and human studies suggest that certain chemical exposures may cause "ototoxic" effects (damage the hearing and balance functions of the ear). In general, the exposure concentrations that cause these effects are considered high. However, exposure to some of these chemicals and noise at the same time can significantly increase the risk of developing ototoxic effects.

What are ototoxins?

Ototoxins are chemicals which can damage hearing and can cause mild to severe hearing loss, tinnitus (ringing in the ears), or deafness. An ototoxin can be ingested, absorbed, or inhaled into the body. Once in the bloodstream, the ototoxin is circulated to the ear and absorbed by the auditory nerve, damaging the nerve and causing hearing loss. Ototoxins can also cause hearing loss by damaging the cochlear hair cells (as happens in hearing loss caused by noise).

Effects of chemical exposure on hearing

Ototoxic chemicals can cause hearing loss on their own, however when combined with noise exposure, the effects can be even more severe. Organic solvents are the most commonly identified chemicals, but others may also be involved (e.g. metals and chemical asphyxiants). The hearing frequencies affected by solvent exposure are different than those affected by noise. Research suggests that solvents may interact synergistically with noise. Even when noise and chemicals are at permissible exposure levels, the impact of a combined exposure can do more damage than a higher exposure to either hazard alone.

Some chemicals associated with hearing loss

Benzene

Carbon disulfide

Carbon monoxide

Ethylbenzene

Hydrogen cyanide

Lead

Mercury

n-Hexane

Solvent mixtures

Styrene

Trichloroethylene

Toluene

Xylene

Organic solvents are widely used: in automotive and aviation fuels; in plastics industries; as thinners for paints, lacquers and dyes; in the manufacture of detergents, medicines, perfumes, fabric and paper coatings, printing inks, spray surface coatings; and in insect repellents.

Activities where noise and chemical hazards can potentially combine include:

boat building

construction

firefighting

fueling vehicles and aircraft

furniture making

manufacturing of metal, leather and petroleum products

painting

printing

weapons firing

Challenges

It may be difficult to determine the ototoxic effects of chemicals, particularly organic solvents, in exposed workers. Workers are usually exposed to a mixture of solvents with various compositions and concentrations, making it difficult to isolate exactly which chemical, and how much exposure to that chemical is causing damage. Also the industrial environments in which there tend to be exposures to both chemicals and high levels of noise make it difficult to differentiate the solvent effect from noise-induced hearing loss.

Although there is no firm guidance on the lowest occupational exposure limits for solvents in relation to their effect on hearing, the current occupational exposure limits as well as hearing conservation programs for solvent-exposed workers may not be adequate.

How to protect workers

Conduct a hazard assessment as the first step in a hearing loss prevention program to learn if and what hazardous exposures exist in the workplace.

Remove the source of hazardous exposures from the workplace (the most effective way to prevent hearing disorders from noise or chemical exposure, but may not be possible).

Wear hearing protection when exposed to noise, or when exposed to ototoxins - even when noise levels are below the threshold - to prevent the combined effects of noise and solvent exposure.

Start a hearing conservation program for workers at lower levels of noise exposure than is required by occupational health and safety legislation.

Include workers exposed to chemicals in hearing conservation programs, whether or not they are exposed to noise. These programs should consider the possible combined effects of exposure to both solvents and noise.

There is a growing awareness of the chemical hazards to hearing that will hopefully lead to reduced risk of work-related hearing loss. Learn more in the CCOHS webinar on October 28th: Chemical Exposure and Hearing Loss in the Workplace, presented by Dr. Thais Morata of NIOSH.

A student accepted a job that was 2.5 hours away - making the commute a 5 hour round trip. The student had worked two day shifts when he was assigned to work the night shift. On his way home after working eight consecutive shifts, the car he was driving left the highway. He was killed in the crash. The police report indicated that he was likely asleep or looking away from the road at the time of the accident.

WorkSafeBC released a human factors bulletin with information on fatigue and how to prevent it in the workplace. Most people need 7.5-8.5 hours of sleep every day, and when they don't get enough, their performance may suffer. They may have slower reaction times and difficulty concentrating and making decisions.

Studies have shown that people working the night shift get about 5 to 7 hours less sleep per week than those working the day shift. The shortened periods of sleep coupled with the natural tendency to be awake in the day means the sleep in the day is not the same quality as night sleep.

In this case, the student's work schedule did not provide enough time to commute and also get enough sleep. He had built up over 12 hours of sleep debt during the eight days he worked. When he was working the six night shifts and sleeping during the day, he was probably not getting the best quality sleep. When he left work to drive home after having been awake for over 19 hours - it is likely that he had a performance level close to that of being legally impaired. The WorkSafeBC bulletin has more data on research that tested continuous hours of wakefulness against blood alcohol levels.

It is important to understand and recognize the signs of fatigue and take preventative steps that minimize the risk of injuries. When developing work schedules, the continuous number of hours and the time of day worked (day vs. night shifts) need to be considered, as well as the potential for accumulated or sudden sleep loss. As part of the management of fatigue, the entire time a worker has to be awake should be taken into account - not just the time spent working (for example, commuting time, especially after working a night shift). This time may add to an already long shift and increase the risk of fatigue impairment.

With falls representing more than 17 per cent of lost-time injuries, Ontario has stepped up efforts to help eliminate injuries and make workplaces safer. The province is conducting a workplace inspection blitz to check for hazards that can cause slips, trips and falls.

Focus of blitz

During the November blitz, Ministry of Labour occupational health and safety inspectors will concentrate on transportation, education, industrial services, retail and wholesale workplaces.

In the past decade, same-level falls (slips and trips) made up about 70 per cent of all lost time fall claims accepted by the Workplace Safety and Insurance Board (WSIB). Floors or other surfaces used by workers must be kept free of obstructions, hazards and accumulations of debris, snow or ice with extra attention given to walkways, aisles, and building exits.

Inspectors will look for fall-from-height hazards involving platforms, raised floors, mezzanines and ladders. They will check fall-arrest equipment and guardrails and will be on the lookout for dangerous practices such as working on storage racks. They will also check for proper use of ladders.

Inspectors will check that requirements are met for supporting workers who are raised on platforms by mobile equipment such as a crane, lift truck or similar equipment. There are specific concerns with reach trucks and fall-distance calculations where lanyards are used at heights below their intended use and could be hazardous to workers.

This workplace inspection blitz is part of the province's Safe At Work Ontario compliance strategy.

With the expected adoption of the Globally Harmonized System of Classification and Labelling of Chemicals (GHS) in Canada, Workplace Hazardous Materials Information System (WHMIS) and other labelling systems will change.

As Canadian federal and provincial regulators work toward adopting GHS, employers and the suppliers of chemicals and other controlled products must prepare to comply with the revised rules for hazard communication. Developed by CCOHS in partnership with Health Canada's National Office of WHMIS, two new free e-courses introduce suppliers, employers and workers to the expected impacts of GHS.

WHMIS after GHS: An Introduction provides a general overview of potential changes to WHMIS after GHS adoption. This 20-minute course also includes links to useful resources such as WHMIS after GHS Infosheets.

Chemical suppliers will want to take the more detailed course WHMIS after GHS : How Suppliers Can Prepare which introduces the impacts on hazard classification, preparing (Material) Safety Data Sheets and labels. This 60-minute course also links to a variety of resources.

The Canadian Centre for Occupational Health and Safety (CCOHS) is calling for entries for the annual Dick Martin Scholarship Award. If you are enrolled in an occupational health and safety programme (that leads to an occupational health and safety certificate, diploma or degree) in a Canadian college or university - you qualify! This year CCOHS will award three scholarships of $3,000 each.

HOW TO APPLY: All you have to do is submit a 1000-1200 word essay on a topic related to your area of study in occupational health and safety. Essays will be judged on the intellectual content, the practical and theoretical value and the presentation and style.